Okay, latest update...I opened up the engine...took off the bowl and the head...im pretty sure that a head gasket was not my problem. I dont know what caused this to happen but some kind of fragments broke off in the cyl and destroyed my piston and the inside of the combustion chamber including the top of the head. The cyl appears to be fine so I am just having it hoaned out but the piston is trashed, I lost compression because one of my rings snapped completely in half( never found where the piston ring fragment went, Im guessing it mostlikely got crushed and sent out the exaust). Guy at the machine shop said the head could just be sanded down with some kind of small grinder and it would not affect the engine. also I didnt hear any clanging in my cyl when it stopped working...could this problem just be really old? and what made it quit working all of the sudden. Any comments?

markm065, on 19 March 2012 - 04:32 PM, said:

My only conclusion is that something big happened when it died. But I've never had such a problem so it's a little over my head. You're lucky to have a machine shop where you are though... Make sure the head gets hoaned correctly, you get a nice new piston (and other stuff), and make sure to dip a magnet down in the crankcase to see if any crud comes up, because the ring may have fallen down there.

My second pistion ring was still there...not enough clearance for it to have gone into the crank case. Yes, I have about 5 different machine shops to choose from haha. In detroit there are many.

markm065, on 20 March 2012 - 05:36 AM, said:

Well good luck, be sure to keep us updated. If you have any questions that I don't know I can ask around.

Well...ive got a big problem now...my piston is too big...the part number matches a std piston from yamaha and i have no idea why it doesnt fit...on the jug it says 246cc which has the same bore as a dt2...my only guess it the cyl has a sleeve in it.

markm065, on 23 March 2012 - 03:50 PM, said:

Did you get around to measuring the actual bore before ordering? That's kinda why I hate the idea of plating the jug because it confuses everything... But yeah, measure everything up, see what you get, and go from there.

Can someone explain to me the relation of pistion sizing and bore...because I dont understand. the bore is 76mm but the piston is .96mm why not just have like a 74mm piston? How can you choose a piston and rings for your cyl.

Okay...so I need a piston for a 70mm bore...its the right size Im an idiot lol...I cant find the part number because yamaha has a wrong number listed...the piston for a dt2 is too big on their site can anyone help me?

markm065, on 24 March 2012 - 03:37 PM, said:

Correct me if I'm wrong but the piston that was in it did work at some point right? Does that piston have any indicators on it? Can you measure it? It's been a little while since I did a top end but you might be looking for a piston about the size of 69.8-9? Find out what size piston you need to fit into the cyl. Then look for a piston that was made for your bike in that size. I know Wiseco's website has all the available sizes. If none of them match your current jug, my uneducated suggestion would be to get it bored to a size that fits the common/std piston size for the bike.

I found this http://www.ebay.com/...r#ht_2634wt_776

I cant tell if its right so Im trying to talk to the company...the site you gave me doesnt list a dt2

markm065, on 25 March 2012 - 01:26 PM, said:

Wow that's really weird they don't even list the size... Correct me if I'm wrong but you do know the std. jug size right? Sorry it's a lazy Sunday for me

std size is 70mm and thats what my bore is

markm065, on 25 March 2012 - 07:43 PM, said:

Oh ok gotcha. Then yeah I'd wait to hear back from the seller of the piston to see if they have any info.

you have about a 50/50 chance of the bottom end being trashed from debris
.
GOOD ADVICE
Before buying a piston kit, Take your cylinder to your local shop. Have them check it and tell you what oversize piston to get  (one over,two over..), then order the piston kit and they will bore and hone to perfectly fit the cylinder to the piston. This usually runs about $60 or so.

This all could have happened from 3 reasons.
!. clogged carb made it run lean
2. timing/points off time
3. crank seals bad creating lean run

Most likely cause for an old bike like this= wore main bearings allowed crank seals to leak (my 75 DT 175 did this twice before i learned the hard way)

Edited by yellowsnow, 27 March 2012 - 07:34 PM.

got everything to rebuild it and it fits this time...cross your fingers...I start first thing tomorrow.

I dont know if it was running lean...no signs on the plugs or on the piston...it had one black spot where the ring broke...but the head wasnt very burned.

markm065, on 29 March 2012 - 04:32 PM, said:

Does that thing still have the original oil tank and oil pump system on it or are you premixing the fuel now? Make sure it's getting enough oil before you try running it again.

I rode and raced a '73 dt250 a few years ago... great little bikes! The last guy had some great advice... make sure its getting oil into the gas. You might just want to be on the safe side and pre-mix some gas instead of trusting the ancient autolube system. I disabled my autolube systems and used premix and rode the snot out of my old beater... kind of regret getting rid of it.



Most parts are readily available through sellers on ebay. The hardest parts to find are crank seals, one is still available, but the other is made of unobtainium and tough to find. there was a guy selling sets on ebay a few years back though. You can also try a local bearing/seal company and see if they can match up a seal thats the right size and application.

Good luck and have FUN on it!

how much oil should come out of the autolube? like a drop per revolution?

ya it's a very small amount. To test in some bike they disconnect the oiler, mix gas, then run the bike for a specific time while catching what the oiler puts out, and measure in CCs.

Edited by yellowsnow, 31 March 2012 - 11:34 AM.